Supermassive black holes and nuclear stellar disks, nuclear keys to galaxy formation and evolution

就像宇宙拼图一样，星系有不同的形状和颜色。了解星系如何演变成今天的性质是天文学中最重要和长期存在的问题之一。在这一领域的科学家中，普遍的观点认为，星系是由恒星形成时期（在此期间，气体变成年轻的蓝色恒星）和合并事件（在此期间，星系碰撞并重新组合成不同的星系结构）的结合形成的。如果星系是同时形成的，它们只会随着恒星数量的增长而变得更红，而不会改变它们的形状。虽然星系的恒星形成历史已经被许多观测所测量，但星系在宇宙时间内合并的速度在很大程度上仍然是未知的。在这方面，天文学家只剩下他们的模型的预测。根据被广泛接受的由冷暗物质主导的宇宙模型，由于星系的质量随着时间的推移而不断增加，所以最大的星系是最近才聚集在一起的，而最小的星系在宇宙历史的早期就已经存在了。此外，群体或星系团中的星系应该比人口密度较低的环境中的星系更早聚集。星系一旦聚集在一起确实很难合并。这是因为他们倾向于以太快的速度相互摇摆，以至于无法合并。作为高级研究员，我将检验这些理论预测。在这项研究中，我将采用通常在星系中心发现的小圆盘中恒星的年龄作为星系合并时间的记录。数值模拟表明，这样的核盘在星系碰撞时很容易被破坏，因此它们在给定星系中的存在表明，自核盘形成以来，没有发生过这样的灾难性事件。如果冷暗物质模型是正确的，我应该会发现核盘是由较小星系中的较老恒星或生活在最拥挤的星系环境中的恒星组成的。到了世纪之交，另一个令人困惑的星系组成部分被证明在星系的形成和演化中起着关键作用：超大质量黑洞。天文学家曾经认为超大质量黑洞只是一种理论上的可能性，但现在已经发现它潜伏在许多星系的中心，包括我们的银河系。最重要的是，中心黑洞的质量似乎与它们的宿主星系的质量成比例，或者只与盘状星系的凸起部分成比例。粗略地说，已经发现中心黑洞的质量几乎比它们的宿主星系或凸起小1000倍。这一惊人的巧合发生在两个大小截然不同的物体之间，超大质量黑洞的大小最多能达到我们太阳系的大小，这有力地表明，超大质量黑洞和星系的演化是密切相关的。然而，这种关系的普遍性尚未确立。这是因为超大质量黑洞的质量只在有限的几个星系中被测量过，而且几乎只在某些星系中被测量过。如果发现以不同方式形成的星系共享相同的黑洞质量内容，那将是令人惊讶的。作为一名高级研究员，我将使用大型地面望远镜完成超大质量黑洞的人口统计，这些望远镜可以借助明亮的参考恒星或强大的激光在高层大气中产生的点源来纠正大气模糊。这将允许我测量超大质量黑洞的质量，就像我使用哈勃太空望远镜一样，并精确观察那些很少有黑洞质量测量的星系。利用这些新数据，我将测试超大质量黑洞和星系之间关系的普遍性，并进一步阐明这种联系的本质。

Like the pieces of a Cosmic Jigsaw, galaxies come in different shapes and colours. Understanding how galaxies evolved their present-day properties is one of the most important and longstanding issues in astronomy. Among the scientists in this field, common wisdom has it that galaxies formed from a combination of star-formation episodes, during which gas turns into young blue stars, and merging events, whereby galaxies collide and reassemble into different galactic configurations. If galaxies formed all at once, they would only become redder as their stellar population ages, without changing their shape. Whereas the star-formation history of galaxies has been measured by many observations, the pace at which galaxies merge through cosmic times remains largely unknown. In this respect, astronomers are left only with the predictions of their models. According to the widely accepted model of a Universe dominated by cold dark matter, because galaxies grow in mass over time by merging together, the largest galaxies have assembled just recently, whereas the smallest galaxies were already in place early on in the history of the Universe. Furthermore, galaxies in groups or clusters should have assembled before their counterparts in less densely populated environments. Galaxies can indeed find it difficult to merge once they crowd together. This is because they tend to swing by each other too fast for them to coalesce. As an Advanced Fellow I will test these theoretical predictions. For this investigation I will adopt as a record of the time when galaxies merged together the age of the stars in the small disks that are often found in the centre of galaxies. Numerical simulations show that such nuclear disks are easily destroyed when galaxies collides, so that their presence in a given galaxy indicates that no such a catastrophic event occurred since the disk formed. If the cold dark matter model is correct, I should find that nuclear disks are made of older stars in smaller galaxies, or in those living in the most crowded galactic environments. By the turn of the century, another puzzling component of galaxies turned out to have a key role in galaxy formation and evolution: Supermassive Black Holes. Once considered by astronomers only as a theoretical possibility, supermassive black holes have now been found to lurk at the center of many galaxies, including our Milky Way. Most importantly, the mass of the central black holes appears to scale with the mass of their host galaxies, or only to that of the bulge component of disk galaxies. Roughly speaking, central black holes have been found to be almost one thousand times less massive than their host galaxy or bulge. This striking coincidence between objects that are vastly different in size, with supermassive black holes extending at most to the size of our Solar system, strongly suggests that the evolution of supermassive black holes and galaxies is intimately related. The universality of this relationship, however, is not yet established. This is because the mass of supermassive black holes has been measured only in a limited number of galaxies, and almost exclusively in certain kinds. It would be surprising if galaxies that formed in different ways were found to share the same black-hole mass content. As an Advanced Fellow, I will complete the demography of supermassive black holes using large ground-based telescopes that can correct for the atmospheric blurring with the aid of bright reference stars or point sources created in the upper atmosphere by powerful lasers. This will allow me to measure the mass of supermassive black holes as if I were using the Hubble space telescope, and to observe precisely those galaxies for which there are very few black-hole mass measurements. Using these new data I will test the universality of the relationship between supermassive black holes and galaxies, and shed more light on the nature of this link.